The present invention relates generally to roller chain and, more particularly, to roller chain pins that are carburized and otherwise heat treated to allow for subsequent coating of a hardened electroless nickel enhanced with a fluorinated carbon as a co-deposit without the sacrifice of requisite metallurgical properties of the roller chain pin.
As will be described in more detail in the detailed description of the present invention, roller chain is normally made up of five components. These components include alternating inside and outside links. The inside links are press fit over bushings and are usually called roller links. The outside links are typically press fit over pins and hence are typically called pin links. Cylindrical rollers are provided outside the bushings leaving the rollers free to turn for a rolling action as the roller chain enters and exists the driving sprockets.
Typically, all high quality components of roller chain, including pins, bushings and rollers, are carburized or case hardened Link plates are thru-hardened. The carburizing process allows the outside of the parts to be transformed to a hard, wear resistant surface whereas the inner core retains the tough and ductile properties of the metal to absorb normal shock loading. In most applications, this combination provides the necessary engineered balance between wear resistance, durability and strength. In efforts to improve the overall performance of roller chain, including improvements in wear life, galling resistance and overall lubricity of the roller chain pins various types of coatings were reviewed. After reviewing the constraints of the design of the roller chain product, performance and manufacturing techniques available, the use of electroless nickel as an autocatalytic deposition was settled on as most desirable to provide corrosion protection on carbon and alloy steel roller chain pins. This was found to be superior to electroplating because of potential for embrittlement in electroplating. Further, tooling treatments such as titanium nitride were viewed as inappropriate due to the manner of application and the labor intensive requirements. Further, flame spraying and ion implantation have similar undesirable restrictions on roller chain manufacture.
Further, a co-deposit of a material to provide lubricity was also desirable. Various components such as silicon carbide, fluorinated carbon and polytetrafluoroethylene were all reviewed. It was decided that the polytetrafluoroethylene or other similar proprietary coatings available today were best suited as a co-deposit with the electroless nickel for the roller chain pins.
The major problem faced in the electroless nickel operation is the need to harden the electroless nickel fluorinated carbon co-deposition at temperatures in the neighborhood of 600.degree.-750.degree. F. to achieve maximum hardness and wear resistance. As most roller chain pins are tempered at 300.degree.-350.degree. F., such subsequent hardening of the electroless nickel coating would result in a reduction of the pin core hardness and strength.